There is a known coding technology that utilizes the concept of a band expansion for an audio signal (see Patent Document 1 and Patent Document 2, for example).
By such a coding technology, a time-series signal that is input as an audio signal is divided into a low-band component and a high-band component. Conventional encoding is performed on the low-band signal, and the relationship between the low-band signal and the high-band signal, the features of the high-band signal, and the like are transmitted as additional information.
Also, at the time of decoding, after the low-band signal is restored, an expanded-band signal is generated with the use of the low-band signal and the additional information. The low-band signal and the expanded-band signal are then combined, to achieve a band expansion.
More specifically, after the low-band signal is restored, the low-band signal is divided into bands by a band dividing filter, and an expanded-band signal is generated with the use of the divided low-band signal and the additional information. The low-band signal and the expanded-band signal are then combined by a band combining filter, and a band-expanded time-series signal is obtained.
Where a band dividing filter and a band combining filter are used as above, however, the fundamental delay between the signal encoding and the decoding is increased due to the filtering processes such as the band dividing process and the band combining process. As a result, the response speed between the input and the output of the audio signal becomes lower.
Further, in addition to the conventional decoding process, the filtering processes such as the band dividing process and the band combining process to be performed by a filter bank or the like become necessary. As a result, the amount of throughput and the memory usage greatly increase, which makes it difficult to install a decoding device in a resource-poor setting, such as an embedded device.
To improve such a coding technology, there is a technology suggested for enabling band expansions in frequency regions (see Patent Document 3, for example).
By this technology, a spectrum obtained through modified discrete cosine transform (MDCT) is divided into the low-band side (baseband) and the high-band side (expanded band) at the time of encoding. Conventional encoding is performed on the baseband signal, and the relationship between the spectrum of the baseband and the spectrum of the expanded band, the features of the spectrum of the expanded band, and the like are transmitted as additional information.
Further, at the time of decoding, the spectrum of the expanded band is generated with the use of the spectrum of the baseband and the additional information, and the spectrum of the baseband and the spectrum of the expanded band are then combined. In this manner, the spectrum of the entire band is generated. Further, inverse modified discrete cosine transform (IMDCT) is performed on the obtained spectrum of the entire band. As a result, the spectrum of the entire band is transformed into a time-series signal (temporal signal).